DMG-09. Targeting the PI3K/AKT/mTOR genetic dependency in combination with ONC201 for the treatment of diffuse midline glioma

Abstract Diffuse midline glioma (DMG) is a highly aggressive pediatric brain cancer. Palliative radiotherapy (RT) is the only approved treatment, with a median overall survival (OS) of less than 11 months. The brain-penetrant oral therapy, ONC201 has shown efficacy in early phase clinical trials, activating the mitochondrial protease ClpP, leading to mitochondrial dysfunction. Our previous studies have shown that DMG cells exposed to ONC201 upregulate alternative energy production pathways via PI3K/mTOR to rescue cell death. Therefore, we combined ONC201 with the brain-penetrant pan-PI3K/mTOR inhibitor paxalisib, with synergistic benefits seen across several immuno-compromised patient derived xenograft (PDX) mouse models, encouraging international Phase II clinical studies (PNOC022/NCT05009992). However, 20% of patients receiving the combination on these trials in the upfront setting, demonstrated immune-related toxicities, including mucositis, colitis and skin irritations. Immune-related toxicities are potentially underpinned by paxalisib’s pan-inhibition of PI3K, as p110γ (PIK3CG) and p110δ (PIK3CD) isoforms are highly expressed in hematopoietic cells, while p110α (PIK3CA) is a genetic dependency specifically in DMG. We hypothesized that the brain penetrant p110α-specific inhibitor, GCT-007, would be an effective alternative to paxalisib, reducing immune-related side effects while maintaining synergism. DMG patient-derived cell lines, showed similar IC50 values for paxalisib and GCT-007 (mean IC50: 0.659 μM and 0.775 μM, respectively). Unbiased proteomic profiling in two patient-derived cell lines exposed to GCT-007, showed significant downregulation of PI3K/AKT/MTOR signaling, validated by immunoblotting. Additionally, pathway analysis identified activation of mitochondrial oxidative phosphorylation, suggesting ONC201 would serve as a synergistic partner with GCT-007. GCT-007 significantly extended OS of our aggressive immunocompetent syngeneic models (median 53 days vs. 40 days, p=0.0001), similar to paxalisib (median 52 days vs. 40 days, p=0.0002). Current studies are investigating GCT-007 combination studies using ONC201 in immune-competent and -compromised models and profiling immune and organ related toxicity to provide necessary safety data for future clinical studies.